Aunus is not to be taken lightly in his first encounter he uses many flame based attacks and he cannot be attacked by any physical attacks as he always has Self Burning, using Waza such as Split Body (Sword), Firecracker (Epee), Double Dragon Wave/Meteor Strike (Spear), Sky Drive/Tomahawk/Yoyo (Axe) or any bow Waza/Magic. He also can charm your team so look out for his charm attack. Self burning is also useful for this battle as he uses Fire Wall which hits all allies in a line, another strategy is to place a character resistant to fire attacks or equipped with armor which resists fire on the front lines to absorb the blunt of the damage using the formations Desert Lance or Tiger's Cave. Aunus will recover HP if the field is covered with the fire element. In his true form the battle can be fought in the same fashion as the first except this time he is much stronger than before so magic like reviver can come in handy here. He also wields the Death Sickle technique which can instantly kill an ally, learning the evasion from enemies like Oannes will help.
If all your characters except for Flurry go down in the first battle with Aunus, Snowman will sacrifice his life by using the eternal ice on him instantly killing him however this results in the death of Flurry and he can never be gotten again after.
Card of Aunas (Abyss Form) from Imperial SaGa
Aunas art featured in the Imperial SaGa x Crystal of Re:Union Collaboration event